Process and apparatus for drying paper



March 26, 1968 J. D. BELLEAU 3,374,550

PROCESS AND APPARATUS on DRYINGPAPER Filed Oct. 20, 1966 2 Sheets-Sheet l 2.5 ISCHMIDTV NUMBER 7 v MoLEcuLAR' WEIGHT I MEDIUM AIR AT IATA. 68). 2

i AMYL ALCOHOL :2.0 LLI m E D Z BUTYL 1" I 9 PROLPRIONIC 1.5 5 F/GJ (D a: ACETIC FORMIC ACID I 6 2O 3O 4O 5O 6O 7O 80 90 I00 MOLECULAR WEIGHT I I I I I 1 I I I I I I I I I I 2 DRYING 0% News PR'INT 4o W UNDER VACUUM I 5 I DRYING o- TWO SIDES a I, I SIZE OF SHEETS=6-49insq' I 1 g INITIAL MOISTURE CONTENT:66.7%- Lu s z 2 I a 35 7 5 2 L g [-76.3 I I Z 5: 25 0.. D O Q 20 20" 'INVE'NTOR 9 k JEAN D.

I BELLEAU I5 o 5 I0 Is 20 25 3o BY TIME MIN Wil m 5 Agent March 26, 1968 J. D. BELLEAU 3,374,550

PROCESS AND APPARATUS FOR DRYING PAPER Filed Oct. 20, 1966 2 Sheets-Sheet 2 x z m I- N Lu 0 5 KS 0) O I 3 o .J a 5 S IO 0 N N T x Us 2 0 z 0 LL! 0 I g INVENTOR JEAN o. BELLEAU BY WM Agent United States Patent 3,374,550 PROCESS AND APFARATUS FOR DRYING PAPER Jean D. Belleau, Ville St. Laurent, Quebec, Canada, as-

signor to Donr nion Engineering Works, Limited, Lachine, Quebec, Canada, a corporation of Canada Filed Oct. 26, 1966, Ser. No. 583,676 Claims. (Ci. 34-9) This invention relates to the treatment of newly formed paper. More particularly it relates to an improved method and apparatus for removing water from a newly formed continuous web of paper, by first replacing a portion of the water by liquid more volatile than water, and then drying the paper by conventional drying apparatus.

The manufacture of paper involves placing a mixture of pulp material and a liquid, normally water, onto a forming wire and subsequently removing the Water, thereby leaving a dry sheet of paper. The water acts as a carrier for the loose pulp fibers, and once the fibers have adhered to each other and taken on the form of a paper sheet, the liquid carrier has served its purpose and must be removed by drying the paper. Generally the newly formed sheet of paper is a continuous web and drying is accomplished by passing the web through a drying station where heat is applied to the web. To obtain the greatest economy in the drying operation it is necessary to keep the energy input, that is, the quantum of energy required to dry a given area of the paper, to a minimum. This is accomplished, for example, by reducing the drying time or temperature.

Accordingly it is an object of this invention to provide a method and apparatus which will allow simplification and economization of the drying step in the formation of a newly formed web of paper.

It has been known generally, heretofore, to decrease the necessary energy input required to dry an object by substituting, for the water in the object, a liquid more volatile than Water, before actually drying the object. This principle has been applied heretofore to improve processes for drying plastic film coatings; and, in the wood pulp industry, for removing Water from loose cellulose in the manufacture of acetate fibers. However, heretofore, no one has discovered a successful procedure for applyin this principle to the manufacture of paper.

For economic and pragmatic reasons paper is generally formed in water. First, water is readily avaiiable so that it is economical. Secondly, paper cannot be formed in the absence of at least some Water. Generally, if a liquid other than water is used, particularly a liquid more volatile than water, the ability of the pulp fibers to adhere to each other to form the paper sheet will be severely impaired. It follows that the paper so formed would have an unacceptably low bonding strength. Consequently, in the past, the thinking has been to refrain from substituting any liquid for the water at any stage in the paper formation process lest the properties of the resultant paper, and particularly the bonding strength, be impaired.

It has been discovered, according to the present invention, however, that the undesirable effects of the more volatile liquids on the resultant paper need not necessairily prohibit the substitution of the more voltalie liquid for the water, at least in part, to enhance drying. Rather, it has been found that under certain conditions, some water may be replaced by the more volatile liquid, to improve operations by decreasing the amount of energy input necessary to dry the paper, while at the same time producing a paper having an acceptable bonding strength.

Specifically, in a referred method of the invention methyl alcohol is substituted for a portion of the water present in the newly formed paper. The substitution step 3,374,559 Patented Mar. 26, 1968 is performed after the Web has passed the forming Wire but before the paper enters the conventional drying apparatus. One would not normally expect to obtain bene ficial results by replacing the water by methyl alcohol since one would anticipate that (1) small amounts of methyl alcohol would have little or no efifect on the drying operation, and (2) larger amounts of methyl alcohol would severly impair the bonding strength of the reslutant paper. It has been found, however, that up to approximately of the water can be replaced by the methyl alcohol before the bonding strength of the resultant paper begins to drop off.

Although, in the preferred method of the invention methyl alcohol is used, it is to be understood that the invention is not limited to a specific substitute liquid. It is only necessary that the liquid be more volatile than water so that the drying procedure will be improved and the strength characteristics of the paper so formed be substantially unetfected. Also, the particular percentage of water replaced by the substitute liquid is not fixed. The type of liquid used and the percentage of water replaced will depend upon many factors, the most important of which are the type of paper being formed, the characteristics desired in the resultant paper, and the operating conditions of the drying step.

In the practice of the invention a newly formed, continuous web of paper is fed to the drying apparatus. Just prior to the actual drying step the web passes over one or more hollow rollers. Spraying apparatus is located immediately adjacent the rollers and adapted to spray a more volatile substitute liquid into the web as the web passes over the roller. At this instant the web becomes saturated with water and the substitute liquid. A vacuum source acts immediately upon the web to draw all excess liquid into the hollow roller. It is desirable that the overall liquid content of the web, by volume, be the same after the hollow roller as before. However, after the roller, part of the liquid content will be the more volatile substitute liquid.

Accordingly, it is an object of this invention to provide an improved method and apparatus for drying paper.

It is another object of this invention to provide a method and apparatus for drying a continuously moving eb of paper using less input energy by substituting for the water a more volatile liquid, while still obtaining a satisfactory paper.

It is another object of this invention to provide a method and apparatus for drying a continuously moving web of paper, originally formed in water, by substituting, for the water a more volatile liquid and then removing the substitute liquid from the paper.

. It is still another object of this invention to provide a method of drying paper in which methyl alcohol is substituted for a portion of the water prior to the drying step.

Other objects and advantages of this invention will become apparent from the detailed description to follow together with the accompanying drawings in which:

FIG. 1 is a graph showing certain properties of several volatile liquids.

FIG. 2 is a diagrammatic illustration of an apparatus of the present invention.

FIG. 3 is a graph illustrating one of the advantages of the present invention, and appears with FIG. 1.

Basically, the drying process involves heat and mass transfer within the material and between its surfaces and the surroundings. This process is governed by rigid laws and it is apparent that, for a given energy input, the properties of the liquid and of the material determine the drying time. According to the present invention the drythe most difiicult liquid to evaporate. For instance, when the free evaporation rate of various liquids was obtained at atmospheric conditions in a relative humidity of 83.0% the following results were obtained:

Liquid Evaporation rate, lb./hr. ft.

Methylene chloride (CH- G ll2.2() 1()"- Other properties relating to evaporation also indicate that water is the most difiicult liquid to evaporate. For example, the volatile liquids listed above were compared with water to determine, under identical conditions, the vapor pressure, the latent heat of vaporization and the surface tension. Water is found to have the lowest vapor pressure, the highest heat of evaporization, and the highest surface tension.

As noted earlier, it has not been considered practical heretofore to replace the water by any other liquid since in other liquids, the paper sheet will not form as readily from the pulp. This result is related to the low dielectric constant of most volatile liquids. The dielectric constant is a measure of the polarity of the liquid and is one of the factors responsible for the bonding strength between the fibers of the resultant paper. The higher the dielectric constant the greater will be the polarity of the liquid and the greater will be the bonding strength which the liquid imparts to the paper. However, those liquids which are the easiest to evaporate have the lowest dielectric constant.

The graph of FIG. 1 illustrates the inverse relationship between the ease with which a liquid evaporates and the ability of the liquid to bind together the pulp fibers to form paper. The points on the graph represent the Schmidt number of various familiar liquids including water, alcohols and acids plotted against the molecular weight of the liquid. The Schmidt number is the viscosity of the air over the density of the air multiplied by the diffusion coetficient of the liquid. This number is a measure of the mass transfer and diffusion ability of the liquid. The dielectric constant e for several of the liquids has also been indicated on the graph. It can readily be seen that water, which, as expected, has the lowest Schmidt number, has the highest dielectric constant and thereby has the greatest ability to bind together the pulp fibers to form paper.

According to a preferred method for carrying out the invention, methyl alcohol is used as the substitute liquid. This liquid is readily available and, as noted in FIG. 1, it has a high dielectric constant compared. to other volatile liquids. However since the dielectric constant of methyl alcohol is still much less than that, for water, one might I expect that it would cause a low bonding strength compared with paper formed exclusively with water. However, this undesirable result would not be expected to occur for very small quantities of methyl alcohol. As the quantity of methyl alcohol is increased to a level at which it has an appreciable effect in reducing the required energy input for drying, one would expect that the quality of the resultant paper would be materially impaired. However, it has been found that the characteristics of the resultant paper does not dropoff proportionately as methyl alcohol is substituted for water. For example, tests have indicated that the product rupture strength remains gen- 4 erally satisfactory until the amount of alcohol reaches about by weight of the total amount of liquid in the newly formed paper. The strength of the resultant paper then falls off rapidly as the percentage of alcohol exceeds 80% and approaches An experiment was conducted to determine the difference in drying rate between a sheet of paper wetted with water and one wetted with pure methyl alcohol. Sheets of paper having an overall area of 6.49 square inches and a thickness of 0.55 and having an initial moisture content of 66.7% were first soaked completely in water and a vacuum source of 20 inches Hg vacuum was applied to the paper to remove the excess water therefrom. The sheets were dried at various drying energy input levels and the times required to fully dry the sheets were recorded. In a second series of runs papers of the same type, size and thickness were soaked completely with water and the excess water was removed therefrom by a vacuum source of 25 inches Hg vacuum. Finally, in the third series of runs, papers were soaked in methyl alcohol and the excess alcohol was withdrawn using only five inches Hg vacuum and the times required to dry the sheets at various drying energy input levels were recorded. The results of these series of runs are shown in the graph of FIG. 3. It can be seen in FIG. 3 that the paper soaked in a pure water solution at a high vacuum took a fairly long time to dry while the paper soaked in the methyl alcohol, even under a very slight vacuum, dried much faster.

FIG. 2 is a schematic drawing of an apparatus for carrying out the present invention on a continuously moving web of paper. A web or" paper 10, having a normal moisture content of 60-70% water, enters the drying apparatus and passes around a roller 11 to a main roller 12. The apparatus may employ either a single main roller 12 or a plurality of main rollers 12 separated by calender rollers 13. As the wet sheet 10 approaches a main roller 12 a substitute liquid in the head 14 is sprayed onto the sheet through nozzles 15. The excess liquid is immediately drawn on by a vacuum source (not shown) acting through opening 16 at the top of the main rollers 12. As the web of paper leaves the last main roller 12 a predetermined amount of water has been replaced by a liquid more volatile than water. Preferably, the overall moisture content of the paper after the last roller 12 is the same as it was prior to the first roller 12. If the liquid content of the web were greater it would place a greater burden on the drying apparatus. A smaller liquid content would of course reduce the load on the drying apparatus. However this savings would be more than offset by the additional input energy to the vacuum source required to obtain the reduction in liquid content at that point. The sheet then passes over press or calender rollers 17, into a conventional predrying area 18 and a conventional drying area 19.

In FIG. 2 the apparatus is shown in housing 20 having a web entrance slot 27 and a web exit slot 28. The enclosed housing is optional but generally desirable from a safety and health point of view and may be used where it is desired to perform the process in a closed loop system to recover the substitute liquid. The use of an inert gas to exclude air and reduce fire hazard is contemplated. In this system excess liquid passes along line 21 to a distillation column 22 and the excess vapor passes along line 23 to the condenser 24. The condenser vapor is then passed into the distillation column 22 where it is distilled alongwith the excess liquid from line 21. The

vapors are then condensed and passed along path 25 to 1. A method for drying a continuously moving web of newly formed paper having a high water content com-' prising the steps of: advancing the web of paper towards a drying station, introducing into the web just prior to the drying station a substitute liquid more volatile than water so that the said liquid replaces a portion of the water originally present in the paper, applying a vacuum to the web simultaneously with the addition of the more volatile liqnuid to remove the excess more volatile liquid and water from the web, and subsequently advancing the web to a drying station and removing substantially all of the remainder of the moisture from the web.

2. A method as claimed in claim 1, wherein the said more volatile liquid is methyl alcohol.

3. A method as claimed in claim 2, wherein said liquid replaces more than 50% of the water present in the newly formed paper just prior to the said introduction step.

4. A method as claimed in claim 2, wherein said liquid replaces approximately 80% of the water present in the newly formed paper just prior to the said introduction step.

5. A method as claimed in claim 2, wherein the aid introducing step is performed by spraying the more volatile liquid onto the continuously moving web as the web passes over a roller, and the excess liquid removal step is performed by a vacuum which acts to draw the excess liquid into the roller.

6. A method as claimed in claim 1, wherein the said introducing step is performed by spraying the more volatile liquid onto the continuously moving web as the web passes over a roller, and the excess liquid removal step is performed by a vacuum which acts to draw the excess liquid into the roller.

7. A method as claimed in claim 1, wherein the moisture content of the web after the spraying and vacuum step is substantially equal to the moisture content of the web prior to the said introducing step.

8. An apparatus for drying a newly formed web of paper having a high water content comprising: a means for advancing a continuously moving web of paper of undeterminate length, a roller positioned so that the web asses thereover, means mounted adjacent the periphery of the roller for introducing a liquid more volatile than water into the web as the web passes over the roller, suction means positioned inside the roller radially adjacent the last said means for removing excess liquid and water from the web as the web passes over the roller, and means for subsequently removing substantially all of the remainder of the moisture in the web after the Web passes over the roller.

9. An apparatus as claimed in claim 8, wherein the said means for introducing the liquid is a Spray mounted to spray the more volatile liquid into the web.

143. An apparatus as claimed in claim 8, including a means for recovering the excess liquid and water removed by the said removing means and returning the said liquid to the said means for introducing liquid.

References Cited UNITED STATES PATENTS 1,687,588 10/1928 Pearson 349 2,060,902 11/1936 Stamrn 34-95 X 2,236,445 3/1941 Pfeiffer 349 2,711,591 6/1955 Nellmar 3418 FREDERICK L. MATTESON, IR., Primary Examiner. J. I. CAMBY, Assistant Examiner. 

1. A METHOD FOR DRYING A CONTINUOUSLY MOVING WEB OF NEWLY FORMED PAPER HAVING A HIGH WATER CONTENT COMPRISING THE STEPS OF: ADVANCING THE WEB OF PAPER TOWARDS A DRYING STATION, INTRODUCING INTO THE WEB JUST PRIOR TO THE DRYING STATION A SUBSTITUTE LIQUID MORE VOLATILE THAN WATER SO THAT THE SAID LIQUID REPLACES A PORTION OF THE WATER ORIGINALLY PRESENT IN THE PAPER, APPLYING A VACUUM TO THE WEB SIMULTANEOUSLY WITH THE ADDITION OF THE MORE VOLATILE LIQUID TO REMOVE THE EXCESS MORE VOLATILE LIQUID AND WATER FROM THE WEB, AND SUBSEQUENTLY ADVANCING THE WEB TO A DRYING STATION AND REMOVING SUBSTANTIALLY ALL OF THE REMAINDER OF THE MOISTURE FROM THE WEB. 